A dielectric lens antenna used in a microwave or millimeter wave band is for refracting an electromagnetic wave which radiates widely from a primary radiator well, aligning the phase thereof on a virtual aperture face ahead of a lens, and also creating an electromagnetic field amplitude distribution on the aperture face thereof. Thus, the electric wave can be made to emit sharply in a certain direction. This dielectric lens antenna resembles a lens used for optics, but the greatest difference is that it is necessary not only to simply align the phase but also to create an amplitude distribution (aperture distribution). This is because antenna properties (directivity) at a distant place have a property represented with the Fourier transform of amplitude distribution, and in order to obtain desired directivity, it is necessary to adjust an aperture distribution well.
Accordingly, it is important with a dielectric antenna, to align the phase of electromagnetic waves over the aperture face, and to create a desired aperture distribution as well.
In order to align the phase over the aperture face, the properties of light rays are utilized wherein even if the distance (light path length) over which the light ray emitted travels, from the primary radiator to the aperture face, changes by an integral multiple of the wavelength, the respective light rays reinforce each other, whereby the shape of the lens can be cut off. This is called zoning. The Fresnel lens, well known for the field of optics, is also based on the same concept as this, but in the case of optics, there is no concept of an aperture distribution.
A dielectric lens antenna comprises a primary radiator such as a horn antenna, and a dielectric lens. In general, the dielectric lens portion of the dielectric lens antenna is high in both weight and volume and in order to reduce the size and weight of the overall equipment, a reduction in the size and weight of the dielectric lens has been desired. As for a method for making a dielectric lens thinner and lighter, the above zoning technique can be employed.
For example, a technique has been disclosed in J. J. Lee, “Dielectric Lens Shaping and Coma-Correction Zoning, Part I: Analysis”, IEEE Transactions on antenna and propagation, pp. 221, vol, AP-31, No. 1, Jan. 1983, (Non-Patent Document 1) wherein an aperture distribution is designed beforehand, following which the rear face side is subjected to zoning, thereby making the aperture distribution after zoning generally equal to that before zoning. FIG. 1 illustrates an example of a dielectric lens which was subjected to zoning. In this drawing, the left side is the side facing a primary radiator (rear face side), and the right side is the side opposite to the primary radiator (surface side).
FIG. 2 is a flowchart illustrating the design method of a dielectric lens of Non-Patent Document 1. First, a desired aperture distribution is determined (S11). Then the center position of the lens, serving as the start point of computations is determined (S12). Subsequently, the solutions of the electric power conservation law, Snell's law regarding a surface (front face), and the formula showing light-path-length constraint, are obtained using numerical computations (S13). Computations are performed for up to the circumferential edges of the lens, to complete the computations of lens shapes which have not been subjected to zoning (S14). Then, the light path length is changed by wavelength at a suitable rear face position along the primary ray, and the rear face shape of the dielectric lens is primarily changed (zoned) (S15). The entire dielectric lens is subjected to this processing of step 15 (S16→S15→and so on).
Also, a technique has been disclosed in Japanese Unexamined Patent Application Publication No. 9-223924 (Patent Document 1) wherein, in order to suppress loss due to refraction caused by zoning, the surface side is made to be a convex shape, and the rear face side is subjected to zoning. FIG. 3 is a cross-sectional view illustrating an example thereof. A dielectric lens 10 forms a recessed portion 2 due to zoning on the rear face side of a dielectric portion 1 (side facing a primary radiator 20).
Also, Richard C. Johnson and Henry Jasik, “Antenna engineering handbook 2nd edition”, McGraw-Hill (1984), (Non-Patent Document 2), a zoning technique for a dielectric lens which had been known by that time in 1984 is described. For example, FIG. 4A is an example wherein the surface side of a dielectric lens has been taken as a plane, with the convex shape on the rear face side subjected to zoning. FIG. 4B is an example wherein the rear face side has been taken as a convex shape, with the plane on the surface side subjected to zoning. Further, FIG. 4C is an example wherein the rear face side has been taken as a plane, with the convex shape on the surface side subjected to zoning.